PHYSIOLOGY OF ATTENTION AND VOLITION. 231 



impossible. If the anterior cerebral arteries, for example, have their 

 supply augmented, then to an exactly corresponding extent a lessened 

 amount can be present in the other encephalic vessels. 



In the second place, no change can occur in its circulation without 

 a change in the balance of active pressure through the brain. The 

 stress through the whole cranial cavity must, of course, be equalized, 

 from the amount of fluids present, but the displacement of solid par- 

 ticles must occur, and such displacement is not likely to be without 

 physiological significance. 



Assuming the approximate soundness of these principles, we have 

 to consider how they may be applied in encephalic physiology. My 

 immediate object will be to show that they must be of essential im- 

 portance in any study of the correlations of mind and brain. 



The first subjective condition or faculty I have to notice on its so- 

 matic side is attention. It is unnecessary to enlarge on the psycho- 

 logical importance of this function. It may be said to underlie every 

 other mental faculty. It is the bringing of the consciousness to a focus 

 in some special direction. It is required to convert sensation into that 

 comprehensive grasp of particulars which constitutes perception ; with- 

 out it, meaningless reverie will take the place of coherent thought ; nor 

 can we conceive of any act being strictly voluntary apart from its 

 guidance. 



To study it in its physiological relation, we may for convenience 

 take the well-known effect of attention in modifying the intensity of 

 sensation. The mental effect produced by an impression on a sensory 

 surface is stronger, and details about the impressing cause are more 

 completely gathered in, when the mind is concentrated on it. On the 

 other hand, if the consciousness is engrossed in some other direction — 

 if absorbed by an interesting occupation or train of thought — the im- 

 pression which formerly produced so much effect is felt obscurely or 

 not at all. To account for this difference we can not be content with 

 a merely metaphysical explanation. To say that the mind is so con- 

 stituted that it can not at one and the same moment entertain with 

 equal distinctness dissociated ideas, is only one half of the truth. 

 There must be a cerebral correlative, and some notion as to the 

 nature of this must be got if we are to come nearer the whole 

 truth. 



Two factors, at least, may be specified as bearing on this problem. 

 In the first place, when the consciousness is engrossed by an immedi- 

 ate sensation, the sphere of encephalic activity is comparatively re- 

 stricted. What that sphere may be in any particular instance it is 

 for anatomy and experiment to determine. For receiving the impres- 

 sion, for quickening the consciousness, and for completing its course 

 as a definite perception, the track involved may be wide and branch- 

 ing, but it does not include the whole brain. 



In the second place, the encephalic circulation will be focused in 



